package fr.free.movierenamer.scrapper.impl.utils;

import java.io.DataInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.nio.LongBuffer;
import java.nio.channels.FileChannel;
import java.nio.channels.FileChannel.MapMode;

/**
 * Hash code is based on Media Player Classic. In natural language it calculates: size + 64bit
 * checksum of the first and last 64k (even if they overlap because the file is smaller than
 * 128k).
 */
public class OpenSubtitlesHasher {

  /**
   * Size of the chunks that will be hashed in bytes (64 KB)
   */
  private static final int HASH_CHUNK_SIZE = 64 * 1024;

  public static String computeHash(File file) throws IOException {
    long size = file.length();
    long chunkSizeForFile = Math.min(HASH_CHUNK_SIZE, size);

    FileChannel fileChannel = new FileInputStream(file).getChannel();

    try {
      long head = computeHashForChunk(fileChannel.map(MapMode.READ_ONLY, 0, chunkSizeForFile));
      long tail = computeHashForChunk(fileChannel.map(MapMode.READ_ONLY, Math.max(size - HASH_CHUNK_SIZE, 0), chunkSizeForFile));

      return String.format("%016x", size + head + tail);
    } finally {
      fileChannel.close();
    }
  }

  public static String computeHash(InputStream stream, long length) throws IOException {

    int chunkSizeForFile = (int) Math.min(HASH_CHUNK_SIZE, length);

    // buffer that will contain the head and the tail chunk, chunks will overlap if length is smaller than two chunks
    byte[] chunkBytes = new byte[(int) Math.min(2 * HASH_CHUNK_SIZE, length)];

    DataInputStream in = new DataInputStream(stream);

    // first chunk
    in.readFully(chunkBytes, 0, chunkSizeForFile);

    long position = chunkSizeForFile;
    long tailChunkPosition = length - chunkSizeForFile;

    // seek to position of the tail chunk, or not at all if length is smaller than two chunks
    while (position < tailChunkPosition && (position += in.skip(tailChunkPosition - position)) >= 0)
      ;

    // second chunk, or the rest of the data if length is smaller than two chunks
    in.readFully(chunkBytes, chunkSizeForFile, chunkBytes.length - chunkSizeForFile);

    long head = computeHashForChunk(ByteBuffer.wrap(chunkBytes, 0, chunkSizeForFile));
    long tail = computeHashForChunk(ByteBuffer.wrap(chunkBytes, chunkBytes.length - chunkSizeForFile, chunkSizeForFile));

    return String.format("%016x", length + head + tail);
  }

  private static long computeHashForChunk(ByteBuffer buffer) {

    LongBuffer longBuffer = buffer.order(ByteOrder.LITTLE_ENDIAN).asLongBuffer();
    long hash = 0;

    while (longBuffer.hasRemaining()) {
      hash += longBuffer.get();
    }

    return hash;
  }

}